Luminescent array, wavelength-converting sealing material and light source

ABSTRACT

The invention proposes an arrangement of luminescent materials for excitation by means of a radiation source and involving the use of a luminescent material having a Ce-activated garnet structure A 3 B 5 O 12 , in which the first component A contains at least one element from the group consisting of Y, Lu, Se, La, Gd, Sm and Tb and the second component B represents at least one of the elements Al, Ga and In, and a plurality of the luminescent materials are mixed together. An associated wavelength-converting casting compound and an associated light-source arrangement are further proposed.

BACKGROUND OF THE INVENTION

The invention concerns an arrangement of luminescent materials, anassociated wavelength-converting casting compound and an associatedlight-source arrangement as set forth in the preambles of claims 1, 4and 9.

It relates in particular to a yellow-emitting or yellow-green-emittingarrangement of garnet luminescent materials for excitation by means ofwavelengths in the blue or near-ultraviolet region of the spectrum. Thecasting compound provided is in particular a cast-resin matrixcontaining the arrangement of luminescent materials, and the lightsource is in particular a light-emitting diode (LED) in combination withthe arrangement of luminescent materials and the casting compound.

A luminescent material for light sources and an associated light sourceare known from WO 98/05078. The light source employed therein is agarnet of the structure A₃B₅O₁₂:D, in which the first component consistsof at least one of various rare-earth metals and component B is one ofthe elements Al, Ga and In. The dopant D is cerium (Ce).

A similar luminescent material in which either Ce or terbium (Tb) isused as a dopant is known from WO 97/50132. Ce emits in the yellowregion of the spectrum, whereas Tb emits in the green. In both cases,the luminescent material is used in combination with a blue-emittinglight source to attain a white mixed color.

A wavelength-converting casting compound based on a luminescent materialknown from the above-cited publications and a transparent casting matrixare known from WO 98/12757. The disclosure content of this document isincorporated herein by reference.

In the production of white mixed light, for example in accordance withWO 97/50132, whose disclosure content, especially with respect to thecomposition and production of the casting, is also incorporated byreference into this description, it is known to vary the colortemperature or the color locus of the white light by appropriate choice,i.e. composition, of the luminescent material, its particle size and itsconcentration. However, optimization of the hue (color locus X and Y inthe CIE chromaticity diagram) of the white light produced is arelatively onerous undertaking. This applies in particular to theso-called achromatic point or “equal energy point” located at thecoordinates CIEX=0.33 and CIEY=0.33.

It is also onerous to optimize the luminescent material for the purposeof achieving better color rendition through a larger proportion of redin the spectrum.

Finally, it is difficult to optimize the luminescent material in termsof its absorption maximum relative to the peak value of the emissionfrom the light emitter.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide anarrangement of luminescent materials of the kind described in theintroduction hereto, which can be produced quickly and simply on thebasis of optimization parameters and is suitable for use with anassociated wavelength-converting casting compound and an associatedlight source.

In accordance with the invention, an arrangement of luminescentmaterials comprising plural luminescent materials is used especiallypreferably for light sources emitting in the short-wave optical regionof the spectrum, especially in the blue or near-ultraviolet spectralregions. Such luminescent materials preferably have a cerium-dopedgarnet structure A₃B₅O₁₂, in which the first component A contains atleast one element from the group consisting of Y, Lu, Se, La, Gd, Sm andTb and the second component B represents at least one of the elementsaluminum, gallium and indium.

The production and mode of action of the described luminescent materialsis described in the publications cited in the introduction hereto.Particularly worth noting in this regard is the fact that. terbium (Tb),when excited in the spectral region between about 400 and 500 nm as aconstituent of the host lattice, i.e., the first component A of thegarnet, is suitable for use as a yellow-emitting luminescent materialwhose dopant is cerium. Terbium has previously been proposed in additionto cerium as an activator for emission in the green region of thespectrum. It is possible to use terbium as the principal constituent ofthe first component A of the garnet, alone or in combination with atleast one of the other rare-earth metals proposed hereinabove.

Especially preferred is a garnet of the structure(Tb_(1−x−y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where

-   -   SE=Y, Gd, La, Sm and/or Lu;    -   0≦x≦0.5−y;    -   0<y<0.1.

At least one of the elements Al and Ga is used as the second component(B). The second component B can additionally contain In. The activatoris cerium.

These luminescent materials absorb electromagnetic radiation with awavelength in the range of 420 nm to 490 nm and can therefore be excitedto irradiate a blue light source, especially a semiconductor LED. GaN-or InGaN-based LED semiconductor chips emitting blue light with anemission maximum in the range of 430 to 480 nm are especially wellsuited for this purpose.

The term “GaN- or InGaN-based light-emitting diode chip” is basically tobe understood, in the context of the present invention, as alight-emitting diode chip whose radiation-emitting region contains GaN,InGaN and/or related nitrides, together with mixed crystals basedthereon, such as Ga(Al,In)N, for example.

Such light-emitting diode chips are known, for example, from ShujiNakamura and Gerhard Fasol, The Blue Laser Diode, Springer Verlag,Berlin/Heidelberg, 1997, pp. 209 et seq.

The previously described luminescent materials are excited by blue lightand in turn emit light whose wavelength is shifted into the range above500 nm. In the case of cerium-activated Tb-garnet luminescent materials,the emission maximum is at about 550 nm.

The above-cited luminescent material absorbs in the range 420 to 490 nmand can thus be excited by the radiation from a blue light source. Goodresults have been obtained with a blue-light-emitting LED chip whoseemission maximum is at 430 to 470 nm. The emission maximum of theTb-garnet:cerium luminescent material is at about 550 nm.

This luminescent material lends itself especially well to use in awhite-light-emitting LED component based on the combination of ablue-light-emitting LED chip with a mixture of luminescent materialsincluding a Tb-garnet-containing luminescent material that is excited bythe absorption of a portion of the emission from the LED chip and whoseemission complements the remaining radiation from the LED to producewhite light.

Especially suitable for use as a blue-light-emitting LED chip is aGa(In)N LED chip, but also any other way of producing a blue LED thatemits in the 420 to 490 nm range. 430 to 470 nm is especiallyrecommended as the principal emission range, since the efficiency ishighest in that case.

The position of the absorption and emission bands of the mixture ofluminescent materials can be finely adjusted through the choice of thetype and quantity of rare-earth metals. The most suitable range for x inthe case of the above-cited Tb-garnet luminescent material when used incombination with light-emitting diodes is

-   -   0.25≦x≦0.5−y.

The especially preferred range for y is 0.02<y<0.06.

Well-suited for use as a component of the luminescent material is agarnet of the structure(Tb_(x)SE_(1−x−y)Ce_(y))₃(Al,Ga)₅O₁₂,

-   -   where SE=Y, Gd, La and/or Lu;    -   0≦x≦0.02, especially x=0.01;    -   0<y<0.1. y is often in the range 0.01 to 0.05.

In general, relatively small amounts of Tb in the host lattice primarilyserve the purpose of improving the properties of known cerium-activatedluminescent materials, while larger amounts of Tb can be addedspecifically to shift the emission wavelength of known cerium-activatedluminescent materials. A high proportion of Tb is therefore especiallywell suited for white LEDs with a low color temperature of less than5000 K.

It is known to use blue-emitting LEDs based on gallium nitride orindium-gallium nitride with emission maxima in the range of 430 to 480nm to excite a luminescent material of the YAG:Ce type, which isdescribed extensively in the literature. Such a luminescent material issold, for example, by the Osram company under the designation L175.Other luminescent materials are known in which the element yttrium (Y)is partially or completely replaced by one of the above-cited rare-earthmetals.

In a luminescent diode suitable for the mixture of luminescent materialsaccording to the invention, the yttrium atoms are for the most partreplaced by terbium. The luminescent material can, for example, have thecomposition (Y_(0.29)Tb_(0.67)Ce_(0.04))₃Al₅O₅, referred to hereinafteras L175/Tb with 67% Tb.

It is provided in accordance with the invention to furnish the hue andthe color locus of the system of luminescent materials by mixingpigmented luminescent-material powders of different compositions andthus different absorption maxima for blue light. This can be done, forexample, by mixing the luminescent material L175 (pure YAG:Ce) with aluminescent material of the described type, in which yttrium ispartially or completely replaced by terbium (L175/Tb, Tb>0%). The ratioof the ingredients can be 1:1. Instead of YAG:Ce, however, it ispossible to use another luminescent material, or another luminescentmaterial produced by modification of said luminescent material, with thefurther option of varying the ratio of the ingredients.

A particular advantage of the invention lies in the fact thatluminescent materials that are available in powdered form can be mixedreadily and therefore permit specific adjustment of the target colorlocus on the CIE chromaticity diagram. Hence, on the chromaticitydiagram, proceeding from a garnet structure such as pure YAG:Ce and thecolor locus of the LED used, a bundle of lines can be plotted, one ofwhich passes through the chosen coordinates of the target color locus.Through the combination of an LED chip and an arrangement of luminescentmaterials, the slope of the resulting color locus line of the individualcolor loci can be varied slightly. It is therefore possible withoutfurther effort to produce a light-source arrangement that includes anLED and a wavelength-converting luminescent material and whose resultingcolor locus line passes exactly through the equal energy point at thecoordinates X=0.33 and Y=0.33 on the color locus diagram. This equalenergy point defines pure white. In addition, a shift in the resultingcolor spectrum, for example in the direction of a higher proportion ofred in the spectrum, which generally results in better color rendition,can be effected by incorporating higher proportions of L175/Tb, forexample.

It is further provided in accordance with the invention to disperse anarrangement of luminescent materials according to the invention in acasting compound that is at least partially transparent to the generatedradiation, preferably in a plastic, especially preferably in, an epoxy,silicone or acrylate casting resin or in a mixture of such resins, or inanother suitable radiation-transmissive material, such as inorganicglass, for example. For this purpose, the arrangement of luminescentmaterials according to the invention is preferably produced as a mixtureof pigment powders with the casting resin and additional elementsaccording to the method disclosed in WO 98/12757.

Further provided in accordance with the invention is a light-sourcearrangement associated with the arrangement of luminescent materials, inwhich a radiation source emits radiation in the blue region or in the UVregion of the optical spectrum and this radiation is partially orcompletely converted into longer-wave radiation by means of thearrangement of luminescent materials according to the invention, theconverted radiation being mixed, in the case of partial conversion, withthe emitted radiation from the radiation source to produce white mixedlight.

Such a light-source arrangement, although comprising only oneluminescent material, is also known from WO 98/12757.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail hereinbelow with reference toan exemplary embodiment in conjunction with FIGS. 1 and 2 of thedrawing, wherein:

FIG. 1 is a color locus diagram showing color locus lines of variousluminescent materials and of the arrangement of luminescent materialsaccording to the invention, and

FIG. 2 is a schematic cross section through the exemplary embodiment ofan arrangement of luminescent materials according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a color locus diagram in which the abscissa is colorlocus coordinate X of the CIE chromaticity diagram and the ordinate iscolor locus coordinate Y.

The plot is based on a light-source arrangement for producing whitemixed light, as described in WO 97/50132, for example.

The LED is, for example, an InGaN-based LED chip that emits in the blueregion of the spectrum and whose color locus point C in the color locuschart is accordingly located at about x=0.14 and y=0.02. Different colorlocus lines are obtained by mixing the blue light from the LED of colorlocus C and the emitted light from a luminescent material, for exampleembedded in a transparent casting resin.

For instance, if pure YAG:Ce is used as the luminescent material, acolor locus line 1 is obtained. When a luminescent material is used inwhich Y is partially or predominantly replaced with terbium, theresulting color locus line passes below color locus line 1. With the useof a luminescent material with a Tb content in the A position of 67%(based on the formula stated hereinabove), color locus line 2 plotted inthe chart is obtained.

Line 1 passes above and line 2 passes below the equal energy point U,which is situated at the color locus coordinates X=0.33 and Y=0.33. Ifthe two luminescent materials yielding color lines 1 and 2 are mixed ina 1:1 ratio and embedded in transparent casting resin (cf. the exemplaryembodiment disclosed below, as depicted in FIG. 2), the result is acolor locus line 3 which, as the diagram of FIG. 1 shows, passes exactlythrough the equal energy point or white point on the color locusdiagram.

In like manner, by mixing various luminescent materials, preferably of agarnet structure, it is possible to obtain color locus curves throughvarious desired coordinates on the CIE chromaticity diagram.

The mixture of luminescent-material powders is advantageously embeddedin a suitably optimized casting resin, it being possible to optimize theparticle sizes of the luminescent-material powders, in particular.Methods for producing such wavelength-converting casting compounds aredescribed in WO 98/12757.

In the especially preferred exemplary embodiment of a light-sourcearrangement illustrated schematically in FIG. 2, a GaN- or InGaN-basedlight-emitting-diode chip 10 is disposed in a recess 11 in a radiopaquebase housing 20, preferably of plastic, for a light-emitting diode.

The base housing 20 for a light-emitting diode is penetrated byelectrical connection paths or legs 21, 22 via which the electricalinterconnections of the chip 10 are led out of the housing.

The inner walls 12 of recess 11 form a reflector for the light emittedby chip 10 and for the light emitted by the mixture of luminescentmaterials, and deflect this light in the direction 13 of maximumradiation of the chip 10.

Recess 11 is filled with a casting compound 14 that comprises atransparent matrix 15 of casting resin, preferably epoxy casting resinor acrylate resin (e.g. polymethyl methacrylate) or a mixture of saidresins, in which the mixture 16 of luminescent-material powders isembedded.

The mixture of luminescent-material powders preferably containsluminescent pigments with particle sizes≦20 μm and a mean particle sized₅₀≦5 μm.

In addition to casting resin 15 and luminescent pigments 16, castingcompound 14 further preferably contains a thixotropic agent, a mineraldiffusor, a water repellent and/or a bonding agent.

In the exemplary embodiment, for example, a white-light-emitting LEDcomponent is present in which the casting compound 14 contains the dyepowders L175 (YAG:Ce) and L175/Tb (with 67% Tb) in a 1:1 ratio and whichemits mixed white light whose color locus is situated on lines 3 in thediagram shown in FIG. 1.

It goes without saying that the explanation of the invention made withreference to the above-described exemplary embodiment is not to beconstrued as a restriction of the invention to the described featuresper se. As the light source, it is possible to use not onlysemiconductor bodies composed of light-emitting-diode chips or laserdiode chips, but also polymer LEDs. Also within the scope of theinvention are luminescent-material powders containing, in addition topure YAG/Ce, fractions of Lu, Se, La, Gd and Sm rather than Y. Furtherincluded are garnets in which the percentage of terbium is lower than inthe above-described formula for a luminescent material.

The arrangement of luminescent materials in accordance with theinvention and the associated casting compound can basically be used withall the designs of light-emitting-diode components disclosed in WO97/50132 and WO 98/12757.

1. A light source arrangement comprising a radiation source that emitsradiation from the wavelength range 400 to 500 nm of the spectrum, and amixture of luminescent materials arranged to receive said radiation,wherein one of said luminescent materials has a Ce-activated garnetstructure having the formula A₃B₅O₁₂, in which the first component Acontains at least one element from the group consisting of Y, Lu, Se,La, Gd, and Sm and the second component B contains at least one elementfrom the group consisting of Al, Ga and In, wherein another of saidluminescent materials has a Ce-activated garnet structure having theformula A₃B₅O₁₂, in which the first component A contains at least oneelement from the group consisting of Y, Lu, Se, La, Gd, Sm and Tb andconsists at least in part of Tb as a constituent of the host lattice,and the second component B contains at least one element from the groupconsisting of Al, Ga and In, wherein said radiation is at leastpartially converted into longer-wave radiation by said mixture ofluminescent materials.
 2. The light source arrangement of claim 1wherein said radiation source is a blue-emitting light-emitting diode.3. The light source arrangement of claim 2 wherein said light-emittingdiode is based on GaN or InGaN.
 4. The light source arrangement of claim3 wherein said mixture of luminescent materials is provided as a mixtureof inorganic luminescent pigment powders that is dispersed in atransparent plastic casting compound that is arranged to receiveradiation from the radiation source.
 5. The light source arrangement ofclaim 4 wherein said luminescent pigment powders have particle sizes≦20μm and a mean particle diameter d₅₀≦5 μm.
 6. The light sourcearrangement of claim 5 wherein said casting compound also includes atleast one member of the group consisting of a thixotropic agent, amineral diffusor, a water repellent and a bonding agent.
 7. The lightsource arrangement of claim 4 wherein said casting compound alsoincludes at least one member of the group consisting of a thixotropicagent, a mineral diffusor, a water repellent and a bonding agent.
 8. Thelight source arrangement of claim 4, 5, 7 or 6 wherein said mixture ofluminescent materials is excitable by radiation from the range of 400 to500 nm.
 9. The light source arrangement of claim 4, 5, 7, or 6 whereinsaid mixture of luminescent materials is excitable by radiation from therange of 420 to 490 nm.
 10. The light source arrangement of claim 4, 5,7, or 6 wherein said another of said luminescent materials includes agarnet structure having the formula(Tb_(1-x-y)SE_(x)Ce_(y))₃(Al,Ga)5O12, where SE=Y, Gd, La, Sm and/or Lu;0≦x≦0.5−y; and 0<y<0.1.
 11. The light source arrangement of claim 4, 5,7 or 6 wherein said another of said luminescent materials includes agarnet structure having the formula(Tb_(1−x−y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, wherein SE=Y, Gd, La, Sm and/orLu; 0≦x≦0.5−y; and 0<y<0.1, and wherein said mixture of luminescentmaterials is excitable by radiation from the range of 400 to 500 nm. 12.The light source arrangement of claim 4, 5, 7 or 6 wherein said anotherof said luminescent materials includes a garnet structure having theformula (Tb_(1−x−y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where SE=Y, Gd, La, Smand/or Lu; 0≦x≦0.5−y; and 0<y<0.1 and wherein said mixture ofluminescent materials is excitable by radiation from the range of 420 to490 nm.
 13. The light source arrangement of claim 1, 2, 3, or 4 whereinsaid radiation is partially converted radiation and is mixed withemitted radiation from said radiation source to produce white light. 14.The light source arrangement of claim 1 wherein said another of saidluminescent materials includes a garnet structure having the formula(Tb_(l−x−y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where SE=Y, Gd, La, Sm and/or Lu;0≦x≦0.5−y; and 0<y<0.1.
 15. A light source arrangement comprising aradiation source that emits radiation from the wavelength range 430 to480 nm of the spectrum, and a mixture of luminescent materials arrangedto receive said radiation, wherein one of said luminescent materials hasa Ce-activated garnet structure having the formula A₃B₅O₁₂, in which thefirst component A contains at least one element from the groupconsisting of Y, Lu, Se, La, Gd, and Sm and the second component Bcontains at least one element from the group consisting of Al, Ga andIn, wherein another of said luminescent materials has a Ce-activatedgarnet structure having the formula A₃B₅O₁₂, in which the firstcomponent A contains at least one element from the group consisting ofY, Lu, Se, La, Gd, Sm and Tb and consists at least in part of Tb as aconstituent of the host lattice, and the second component B contains atleast one element from the group consisting of Al, Ga and In, whereinsaid radiation is at least partially converted into longer-waveradiation by said mixture of luminescent materials.
 16. A light sourcearrangement comprising a radiation source that emits radiation from thewavelength range 400-500 nm of the spectrum, and a mixture ofluminescent materials having different compositions arranged to receivesaid radiation, wherein at least one of said luminescent materials has aCe-activated garnet structure having the formula A₃B₅O₁₂, in which thefirst component A contains at least one element from the groupconsisting of Y, Lu, Se, La, Gd, Sm, and Tb and consists at least inpart of Tb as a constitute of the host lattice, and the second componentB contains at least one element from the group consisting of Al, Ga andIn, wherein said radiation is at least partially converted intolonger-wave radiation by said mixture of luminescent materials.
 17. Alight source arrangement comprising a radiation source that emitsradiation from the wavelength range 400 to 500 nm of the spectrum, and amixture of luminescent materials arranged to receive said radiation,wherein one of said luminescent materials has a Ce-activated garnetstructure having the formula A₃B₅O₁₂, in which the first component Acontains at least one element from the group consisting of Y, Lu, La,Gd, and Sm and the second component B contains at least one element fromthe group consisting of Al, Ga and In, wherein another of saidluminescent materials has a Ce-activated garnet structure having theformula A₃B₅O₁₂, in which the first component A contains at least oneelement from the group consisting of Y, Lu, La, Gd, Sm and Tb andconsists at least in part of Tb as a constituent of the host lattice,and the second component B contains at least one element from the groupconsisting of Al, Ga and In, wherein said radiation is at leastpartially converted into longer-wave radiation by said mixture ofluminescent materials.
 18. The light source arrangement of claim 17wherein said radiation source is a blue-emitting light-emitting diode.19. The light source arrangement of claim 18 wherein said light-emittingdiode is based on GaN or InGaN.
 20. The light source arrangement ofclaim 18 wherein said mixture of luminescent materials is provided as amixture of inorganic luminescent pigment powders that is dispersed in atransparent plastic casting compound that is arranged to receiveradiation from the radiation source.
 21. The light source arrangement ofclaim 20 wherein said luminescent pigment powders have particle sizes≦20μm and a mean particle diameter d₅₀≦5μm.
 22. The light sourcearrangement of claim 21 wherein said casting compound also includes atleast one member of the group consisting of a thixotropic agent, amineral diffusor, a water repellent and a bonding agent.
 23. The lightsource arrangement of claim 20 wherein said casting compound alsoincludes at least one member of the group consisting of a thixotropicagent, a mineral diffusor, a water repellent and a bonding agent. 24.The light source arrangement of claim 20, wherein said mixture ofluminescent materials is excitable by radiation from the range of 400 to500 nm.
 25. The light source arrangement of claim 20, wherein saidmixture of luminescent materials is excitable by radiation from therange of 420 to 490 nm.
 26. The light source arrangement of claim 20,wherein said another of said luminescent materials includes a garnetstructure having the formula (Tb_(1-x-y)SE_(x)Cey)₃(Al,Ga)₅O₁₂, whereSE=Y, Gd, La, Sm and/or LU; 0≦×≦0.5-y; and 0<y<0.1.
 27. The light sourcearrangement of claim 20, wherein said another of said luminescentmaterials includes a garnet structure having the formula(Tb_(1-x-y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where SE=Y, Gd, La, Sm and/or Lu;0≦×≦0.5-y; and 0<y<0.1, and wherein said mixture of luminescentmaterials is excitable by radiation from the range of 400 to 500 nm. 28.The light source arrangement of claim 20, wherein said another of saidluminescent materials includes a garnet structure having the formula(Tb_(1-x-y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where SE=Y, Gd, La, Sm and/or Lu;0≦×≦0.5-y; and 0<y<0.1, and wherein said mixture of luminescentmaterials is excitable by radiation from the range of 420 to 490 nm. 29.The light source arrangement of claim 17 wherein said another of saidluminescent materials includes a garnet structure having the formula(Tb_(1-x-y)SE_(x)Ce_(y))₃(Al,Ga)₅O₁₂, where SE=Y, Gd, La, Sm and/or Lu;0≦×≦0.5-y; and 0<y<0.1.
 30. The light source arrangement of claim 17wherein said radiation is partially converted radiation and is mixedwith emitted radiation from said radiation source to produce whitelight.
 31. A light source arrangement comprising a radiation source thatemits radiation from the wavelength range 430 to 480 nm of the spectrum,and a mixture of luminescent materials arranged to receive saidradiation, wherein one of said luminescent materials has a Ce-activatedgarnet structure having the formula A₃B₅O₁₂, in which the firstcomponent A contains at least one element from the group consisting ofY, Lu, La, Gd, and Sm and the second component B contains at least oneelement from the group consisting of Al, Ga and In, wherein another ofsaid luminescent materials has a Ce-activated garnet structure havingthe formula A₃B₅O₁₂, in which the first component A contains at leastone element from the group consisting of Y, Lu, La, Gd, Sm and Tb andconsists at least in part of Tb as a constituent of the host lattice,and the second component B contains at least one element from the groupconsisting of Al, Ga and In, wherein said radiation is at leastpartially converted into longer-wave radiation by said mixture ofluminescent materials.
 32. A light source arrangement comprising aradiation source that emits radiation from the wavelength range 400-500nm of the spectrum, and a mixture of luminescent materials havingdifferent compositions arranged to receive said radiation, wherein atleast one of said luminescent materials has a Ce-activated garnetstructure having formula A₃B₅O₁₂, in which the first component Acontains at least one element from the group consisting of Y, Lu, La,Gd, Sm, and Tb and consists at least in part of Tb as a constitute ofthe host lattice, and the second component B contains at least oneelement from the group consisting of Al, Ga and In, wherein saidradiation is at least partially converted into longer-wave radiation bysaid mixture of luminescent materials.